Chrome plated parts and chrome plating method

ABSTRACT

Using a chrome plating bath containing organic sulfonic acid, plating is conducted by application of a pulse current to thereby form a crack-free lower chrome layer on a steel substrate. The lower chrome layer has a compressive residual stress of 100 MPa or more and a crystal grain size of from 9 nm to less than 16 nm. Subsequently, by application of a direct current, a cracked upper chrome layer is formed on the lower chrome layer, to thereby obtain a chrome plated part. The lower chrome layer imparts the chrome plated part with heat resistance and corrosion resistance, and the upper chrome layer imparts the chrome plated part with wear resistance and good sliding properties.

BACKGROUND OF THE INVENTION

The present invention relates to chrome plated parts comprisingsubstrates having industrial chrome plating applied on the surfacesthereof. The present invention also relates to a chrome plating methodand a production method for obtaining such parts.

Chrome plating, especially hard chrome plating, provides a hard metalliccoating (i.e., a chrome layer) having a low coefficient of friction.Therefore, chrome plating has been widely used as industrial chromeplating for parts which are required to have high wear resistance.

With respect to general-purpose hard chrome plating, a chrome layerformed on a metallic substrate contains many cracks reaching thesubstrate, called channel cracks. Such a chrome layer enables acorrosive material to migrate into the metallic substrate and causecorrosion. This leads to formation of red rust when the substrate ismade of steel.

In producing chrome plated parts, generally, a plated substrate issubjected to polishing, such as buffing, so as to provide a smoothsurface. It is known that during polishing, cracks in a chrome layerbecome clogged due to the occurrence of plastic flow over the surface ofthe chrome layer. Therefore, in producing general-purpose chrome platedparts, after polishing, no special measures have been taken to preventrusting.

However, when a chrome layer is subject to thermal hysteresis,contraction of the chrome layer occurs. In this case, cracks which havebeen clogged due to plastic flow in the chrome layer are caused to open.Consequently, parts which are used at temperatures higher than roomtemperature (for example, at 120° C. for 100 hours or more) are likelyto suffer a lowering in corrosion resistance.

As a countermeasure, it has been attempted to conduct nickel plating orcopper plating as a pretreatment, to thereby form a lower layer having athickness almost equal to that of a chrome layer to be formed, andconducting hard chrome plating on the lower layer. However, in thiscountermeasure, a plating process must be conducted in two steps,leading to low productivity and high process costs.

As another countermeasure, it has been proposed to conduct chromeplating by using two different plating baths, to thereby deposit twochrome layers having different crystal orientations, thus preventing theformation of cracks reaching the substrate [reference is made to, forexample, Unexamined Japanese Patent Application Public Disclosure(Kokai) No. 4-350193]. However, this countermeasure also requires atwo-step plating process.

Further, there is a method of conducting electro-plating with a pulsecurrent, so-called pulse plating, so as to obtain a crack-free chromelayer [reference is made to, for example, Unexamined Japanese PatentApplication Public Disclosure (Kokai) No. 3-207884]. However, the chromelayer formed simply by pulse plating is subject to tensile residualstress. This leads to the formation of large cracks in the chrome layerdue to the application of heat.

Further, there is a method of conducting pulse plating in a Sargent bathby application of an irregular pulse current, to thereby obtain acrack-free decorative chrome layer [reference is made to, for example,Examined Japanese Patent Application Publication (Kokoku) No. 43-20082].The chrome layer obtained by this method has low (or no) stress.However, the obtained chrome layer has a stress gradient (as thethickness of the chrome layer becomes large, the value of stress shiftsfrom a side of compressive stress toward a side of tensile stress).Therefore, average compressive stress in the chrome layer is undesirablylow. Consequently, when the above-mentioned chrome layer is used as alower layer and a cracked chrome layer is formed as an upper layer byplating on the lower chrome layer, the lower chrome layer is subject totensile stress from the upper chrome layer, so that propagation ofcracks through the upper chrome layer to the lower chrome layer occurs.Further, in the chrome plating bath in Kokoku No. 43-20082, averagecompressive residual stress can be increased only to a level as low as100 MPa, even by controlling the waveform of an applied pulse current, abath temperature and a current density.

In view of the above, the present invention has been made. It is anobject of the present invention to provide chrome plated parts whichmaintain excellent corrosion resistance even when the chrome platedparts are subject to thermal hysteresis. It is another object of thepresent invention to provide a chrome plating method and a productionmethod for efficiently obtaining such chrome plated parts.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a chrome platedpart comprising a substrate having a crack-free chrome layer applied ona surface thereof. The crack-free chrome layer has compressive residualstress and is formed by plating.

In the chrome plated part of the present invention in which a crack-freechrome layer having compressive residual stress is formed on a surfaceof the substrate, due to the compressive residual stress in the chromelayer, no formation of cracks in the chrome layer occurs. Therefore, thechrome layer maintains a crack-free structure. Consequently, the chromeplated part maintains excellent corrosion resistance even when it issubject to thermal hysteresis.

When compressive residual stress in the chrome layer is too low, thecompressive residual stress changes to tensile residual stress due tothe occurrence of thermal hysteresis. This leads to the formation ofcracks in the chrome layer. Therefore, it is preferable for thecompressive residual stress in the crack-free chrome layer to be 100 MPaor more.

Generally, when a chrome layer is subject to thermal hysteresis, theformation of cracks is likely to occur due to contraction of the chromelayer. This contraction is affected by the amount of lattice defectspresent in crystal grain boundaries in the chrome layer. Therefore,contraction of the chrome layer due to thermal hysteresis can besuppressed by suppressing the amount of lattice defects, that is, byincreasing a crystal grain size and decreasing the length of a crystalgrain boundary (the length of a crystal grain boundary is in inverseproportion to a crystal grain size). Therefore, in the chrome platedpart of the present invention, it is preferred that the crystal grainsize of the crack-free chrome layer be 9 nm or more.

The crystal grain size of a chrome layer formed by general-purpose hardchrome plating is as small as about 6 nm. The above-mentioned crystalgrain size of the chrome layer in the present invention is much largerthan this size. Therefore, the chrome layer in the present inventioncontains no cracks even prior to polishing, and maintains a crack-freestructure even when it is subject to thermal hysteresis. Therefore, thechrome plated part has desired corrosion resistance. When the crystalgrain size is too large, a crystal structure of the chrome layerchanges. Therefore, it is preferable for the crystal grain size of thecrack-free chrome layer to be less than 16 nm.

In the chrome plated part of the present invention, the crack-freechrome layer may be a lower chrome layer and the chrome plated part mayfurther comprise a cracked upper chrome layer which is formed or appliedon the lower chrome layer by plating. In this case, the hardness of theupper chrome layer can be increased to a maximum level. This improveswear resistance of the chrome plated part. Further, cracks in the upperchrome layer serve as oil sumps for holding lubricating oil, leading tosuppression of sliding resistance.

The chrome plated part may further comprise at least one intermediatechrome layer which is formed between the lower chrome layer and theupper chrome layer by plating. When an intermediate chrome layer isprovided, direct propagation of cracks through the upper chrome layer tothe lower chrome layer can be suppressed. Therefore, corrosionresistance of the chrome plated part can be stably maintained.

The chrome plated part may further comprise an oxide film containingCr₂O₃ as an outermost layer thereof. In this case, the chrome layeritself has high corrosion resistance, so that formation of white rustcan be prevented.

The present invention also provides a chrome plating method comprisingthe step of conducting electroplating of a work in a chrome plating bathby application of a pulse current, the chrome plating bath containingorganic sulfonic acid, to thereby deposit a crack-free chrome layer on asurface of the work. The crack-free chrome layer has compressiveresidual stress.

In the chrome plating method of the present invention, by adjusting apulse waveform of an applied current which alternates between a maximumcurrent density and a minimum current density, the compressive residualstress and crystal grain size of a chrome layer can be easilycontrolled. Therefore, it is possible to obtain a chrome layer having acompressive residual stress of 100 MPa or more and a crystal grain sizeof from 9 nm to less than 16 nm.

In the chrome plating method of the present invention, theabove-mentioned chrome layer may be formed as a lower chrome layer andthe above-mentioned upper chrome layer or the above-mentionedintermediate and upper chrome layers may be formed on the lower chromelayer. In this case, after the chrome layer is deposited as a lowerchrome layer by using the pulse plating, electroplating of the work isconducted in the same chrome plating bath as the chrome plating bath forthe pulse plating, by one of adjustment of a waveform of the pulsecurrent and application of a direct current, to thereby deposit theupper chrome layer or intermediate chrome layer efficiently.

The chrome layers may be deposited by continuous operation bycontinuously moving the work in the chrome plating bath or may bedeposited by batchwise operation by immersing the work in the chromeplating bath.

Further, the present invention provides a method for producing a chromeplated part, comprising the steps of: conducting the above-mentionedchrome plating method for the two or more than three layers; polishingthe upper surface of the work; and conducting heat oxidation, to therebyform an oxide film containing Cr₂O₃ on a surface of the chrome layer.

When the upper chrome layer containing cracks is formed by the chromeplating method of the present invention, the cracks in the upper chromelayer become clogged during polishing due to the above-mentioned plasticflow in the chrome layer. Although the cracks are caused to open againdue to heat oxidation after polishing, the chrome plated part hassufficient corrosion resistance for preventing formation of red rust,because the crack-free lower chrome layer is present on the substrate.In addition, since an oxide film containing Cr₂O₃ is present as theoutermost layer of the chrome plated part, corrosion of the chrome layeritself can be suppressed, thus preventing formation of white rust.

In the method of the present invention for producing a chrome platedpart, the method of heat oxidation is not particularly limited. Forexample, heat oxidation can be conducted under the same conditions asconditions of a general-purpose baking process or by high-frequencyheating. With respect to the general-purpose baking process, FederalSpecification QQ-C-320a(1967. 7. 25) requires that when steel having ahardness of HRC 40 or more is used as a substrate, the baking process beconducted at 191±14° C. for 3 hours or more. By conducting heatoxidation under the above-mentioned conditions, an oxide film containingCr₂O₃ is formed on a surface of a substrate. As a method of heatoxidation by high-frequency heating, for example, a substrate is held ata temperature as high as about 400° C. for a short period of time offrom several seconds to several tens of seconds.

The foregoing and other objects, features and advantages of the presentinvention will be apparent from the following detailed description andappended claims taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of cross section showing a surfacestructure of a chrome plated part according to a first embodiment of thepresent invention.

FIG. 2 is a graph showing an example of a waveform of a pulse current ina chrome plating process for obtaining the chrome plated part of FIG. 1.

FIG. 3 is a top view schematically showing a structure of a platingapparatus used in the method of the present invention.

FIG. 4 is a schematic illustration showing a surface structure of achrome plated part according to a second embodiment of the presentinvention.

FIG. 5 is a graph showing an example of a waveform of a pulse current ina chrome plating process for obtaining the chrome plated part of FIG. 4.

FIG. 6 is a schematic illustration showing a surface structure of achrome plated part according to a third embodiment of the presentinvention.

FIG. 7 is a top view schematically showing a structure of a systemincluding polishing and heating apparatuses for obtaining the chromeplated part of FIG. 6.

FIG. 8 is a microphotograph showing white rust formed in Examples.

FIG. 9 is a graph showing a relationship between a thickness of platingand residual stress in the chrome plated part of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinbelow, embodiments of the present invention are explained, withreference to the drawings.

FIG. 1 shows a chrome plated part according to a first embodiment of thepresent invention. The chrome plated part comprises: a steel substrateM; a crack-free lower chrome layer S₁ formed by plating on a surface ofthe substrate M; and a multicracked upper chrome layer S₂ formed byplating on the lower chrome layer S₁. The cracks in the chrome layer S₂are designated by a reference character F. The lower chrome layer S₁ hasa compressive residual stress of 100 MPa or more and has a crystal grainsize of from 9 nm to less than 16 nm. The upper chrome layer S₂ has acompressive residual stress less than 100 MPa or a tensile residualstress and has a crystal grain size less than 9 nm.

In the above-mentioned chrome plated part, the crack-free lower chromelayer S₁ is present below the upper chrome layer S₂. Therefore, althoughthe cracks F are present in the upper chrome layer S₂, a corrosivematerial does not migrate into the substrate M, so that a desiredcorrosive resistance of the chrome plated part can be ensured. Further,the lower chrome layer S₁ has a predetermined compressive residualstress and a predetermined crystal grain size, so that the lower chromelayer S₁ maintains a crack-free structure even when it is subject tothermal hysteresis, to thereby ensure excellent corrosion resistance ofthe chrome plated part. In addition, since the upper chrome layer S₂ maycontain cracks such as the cracks F, the hardness of the upper chromelayer S₂ can be increased to a sufficiently high level (900 HV or more),to thereby impart the chrome plated part with sufficient wearresistance. Further, the cracks F present in the upper chrome layer S₂serve as oil sumps for holding lubricating oil, which enhances slidingproperties of the chrome plated part.

The chrome layers S₁ and S₂ are formed by a two-step plating process ina chrome plating bath containing organic sulfonic acid. The two-stepplating process comprises plating utilizing a pulse current(hereinafter, frequently referred to as “pulse plating”) and platingutilizing a direct current (hereinafter, frequently referred to as“general-purpose plating”). An example of a current density pattern ofan applied current for this process is shown in FIG. 2.

As the chrome plating bath containing organic sulfonic acid, it ispreferred to use a chrome plating path described in Examined JapanesePatent Application Publication (Kokoku) No. 63-32874, which hascompositions as shown in Table 1.

TABLE 1 Amount (g/L) Component Suitable Preferable Chromic acid 100-450200-300 Sulfuric acid 1-5 1.5-3.5 Organic sulfonic acid  1-18 1.5-12 Boric acid  0-40  4-30

Referring to FIG. 2, a zone A indicates a region for the pulse platingfor forming the chrome layer S₁ and a zone B indicates a region for thegeneral-purpose plating for forming the upper chrome layer S₂. In thezone A, the applied current alternates between two current densities,namely, a maximum current density I_(U) and a minimum current densityI_(L). The maximum current density I_(U) is held for a predeterminedtime period T₁ and the minimum current density I_(L) is held for apredetermined time period T₂. In the example of FIG. 2, the minimumcurrent density I_(L) is set to zero (off). However, needless to say,the minimum current density I_(L) may be arbitrarily set to a valuebetween the maximum current density I_(U) and zero. Further, the valuesof the time periods T₁ and T₂ may be set as being the same or different.In the first embodiment, for pulse plating, the maximum current densityI_(U), the minimum current density I_(L) (I_(L)=0 in this example), thetime period T₁ at the maximum current density I_(U) and the time periodT₂ at the minimum current density I_(L) are set to appropriate values,to thereby obtain the lower chrome layer S₁ (FIG. 1) having apredetermined compressive residual stress and a predetermined crystalgrain size.

FIG. 3 shows an example of an apparatus for obtaining a chrome platedpart having the above-mentioned two chrome layers S₁ and S₂. In FIG. 3,works (such as piston rods) W are suspended from endlessly movablehangers 1. A mounting station 2, an alkali electrolytic degreasing tank3, a plating tank 4, a cleaning tank 5 and a removing station 6 arearranged in this order below a line of movement of the hangers 1. Theplating tank 4 comprises an etching process tank 4A disposed adjacent tothe alkali electrolytic degreasing tank 3 and a plating process tank 4Badjacent to the etching process tank 4A. The plating process tank 4Bcontains the above-mentioned chrome plating bath containing organicsulfonic acid.

Separate bus bars 7, 8 and 9 are arranged along the alkali electrolyticdegreasing tank 3, the etching process tank 4A and the plating processtank 4B, respectively. The bus bar 9 extending along the plating processtank 4B comprises a front bus bar 9A on a side of the etching processtank 4A and a rear bus bar 9B on a side of the cleaning tank 5. The busbar 7 corresponding to the alkali electrolytic degreasing tank 3, thebus bar 8 corresponding to the etching process tank 4A and the rear busbar 9B corresponding to the plating process tank 4B are connected todirect current sources 10, 11 and 13, respectively. The front bus bar 9Acorresponding to the plating process tank 4B is connected to a pulsecurrent source 12.

The hangers 1 have feeding brushes 14. The feeding brushes 14 arebrought into sliding contact with the bus bars 7, 8, 9A and 9B, so thata current is equally applied from the current sources 10, 11, 12 and 13to each of the hangers 1. In each of the alkali electrolytic degreasingtank 3 and the etching process tank 4A, a plurality of cathodesconnected in parallel are provided. The cathodes in the alkalielectrolytic degreasing tank 3 and the cathodes in the etching processtank 4A are designated by reference numerals 15 and 16, respectively.The plating process tank 4B contains a plurality of anodes 17corresponding to the front bus bar 9A, which are connected in parallel,and a plurality of anodes 18 corresponding to the rear bus bar 9B, whichare also connected in parallel. The current sources 10 and 11 applycurrents to the corresponding cathodes 15 and 16, and the currentsources 12 and 13 apply currents to the corresponding anodes 17 and 18.In the plating process tank 4B, ammeters 19a and 19b are providedbetween the anode 17 and the current source 12 and between the anode 18and the current source 13, respectively.

In order to conduct a chrome plating process using the above-mentionedapparatus, the works W are mounted on the hangers 1 in the mountingstation 2. The works W are moved successively to the alkali electrolyticdegreasing tank 3 and the etching process tank 4A while being suspendedfrom the hangers 1. In the alkali electrolytic degreasing tank 3, adegreasing process is conducted while making the works W anode. In theetching process tank 4A, an etching process is conducted while makingthe works W anode. Subsequently, the works W are moved to the platingprocess tank 4B, where a chrome plating process is conducted whilemaking the works W cathode.

In the chrome plating process, a current having a pulse waveform, suchas that indicated in the zone A of FIG. 2, is applied from the currentsource 12 to the works W through the front bus bar 9A and the anodes 17,to thereby conduct pulse plating. Pulse plating is continued while thefeeding brushes 14 of the hangers 1 (from which the works W aresuspended) are in contact with the front bus bar 9A. Consequently, thecrack-free lower chrome layer S₁ (FIG. 1) is formed on a surface of eachwork W. Subsequently, the feeding brushes 14 of the hangers 1 (fromwhich the works W are suspended) move onto the rear bus bar 9B, andgeneral-purpose plating is conducted by application of a direct currentfrom the current source 13 to the works W through the rear bus bar 9Band the anodes 18. General-purpose plating is continued while thefeeding brushes 14 of the hangers 1 (from which the works W aresuspended) are in contact with the rear bus bar 9B. Consequently, themulticracked chrome layer S₂ having the cracks F is formed on the lowerchrome layer S₁ in a superimposed manner as shown in FIG. 1. Thereafter,the works W are cleaned with water in the cleaning tank 5 and moved tothe removing station 6, where the works W are removed from the hangers1.

In the above-mentioned chrome plating process, the two chrome layers S₁and S₂ can be formed by continuously moving the works W in the samechrome plating bath. Therefore, chrome plated parts having excellentcorrosion resistance and heat resistance can be produced efficiently.

In the above-mentioned embodiment, hard chrome plating is conducted intwo steps so as to form the two chrome layers S₁ and S₂. However, in thepresent invention, the upper chrome layer S₂ may be omitted and only thechrome layer S₁ may be formed on the work W. In this case, thecrack-free chrome layer S₁ is exposed to the outside and there is no oilsump for holding lubricating oil as in the case of the upper chromelayer S₂ being formed on the lower chrome layer S₁. However, the chromelayer S₁ is satisfactory in terms of corrosion resistance.

Further, in the first embodiment, the lower chrome layer S₁ and theupper chrome layer S₂ are formed by continuous operation using theapparatus shown in FIG. 3. However, in the present invention, a singleplating tank containing a chrome plating bath may be prepared and thelower chrome layer S₁ and the upper chrome layer S₂ may be formed bybatchwise operation using this plating tank. In this case, an output ofa current source is controlled by means of a controller so that adesired current density pattern of an applied current, such as thatshown in FIG. 2, can be obtained.

For batchwise operation, instead of using a single plating tank, aplating tank for forming the lower chrome layer S₁ and a plating tankfor forming the upper chrome layer S₂ may be separately provided, andthe lower chrome layer S₁ and the upper chrome layer S₂ may be formed byapplying a pulse current to the plating tank for forming the lowerchrome layer S₁ and applying a direct current to the plating tank forforming the upper chrome layer S₂.

FIG. 4 shows a chrome plated part according to a second embodiment ofthe present invention. A feature of this embodiment resides in that twointermediate chrome layers S₃ and S₄ are provided between the lowerchrome layer S₁ and the upper chrome layer S₂. The properties of theintermediate chrome layers S₃ and S₄ are not particularly limited.However, it is preferred that the intermediate chrome layer S₃ on a sideof the lower chrome layer S₁ has properties similar to those of thelower chrome layer S₁ and the intermediate chrome layer S₄ on a side ofthe upper chrome layer S₂ has properties similar to those of the upperchrome layer S₂. Therefore, a few cracks F may be present in theintermediate chrome layer S₄.

By providing the intermediate chrome layers S₃ and S₄ between the lowerchrome layer S₁ and the upper chrome layer S₂, direct propagation ofcracks from the upper chrome layer S₂ to the lower chrome layer S₁ canbe suppressed, so that corrosion resistance of the chrome plated partcan be stably maintained. Although the two intermediate layers S₃ and S₄are provided in this embodiment, the number of intermediate chromelayers is not specifically limited in the present invention. A singleintermediate layer or three or more intermediate layers may be provided.

A chrome plated part in the second embodiment of the present inventioncan be obtained by, for example, setting zones C₁ and C₂ between theabove-mentioned zones A and B (FIG. 2) as shown in FIG. 5 and setting awaveform of a pulse current in the zones C₁ and C₂ to the patterndifferent from that in the zone A. With respect to an apparatus forobtaining the chrome plated part in the second embodiment, substantiallythe same apparatus as the apparatus of FIG. 3 can be used, except thatthe front bus bar 9A (FIG. 3) corresponding to the plating process tank4B is divided into a plurality of bus bars which are connected todifferent pulse current sources 12.

FIG. 6 is a chrome plated part according to a third embodiment of thepresent invention. A feature of this embodiment resides in that an oxidefilm S₅ containing Cr₂O₃ as a main component is formed as an outermostlayer of the chrome plated part. The oxide film S₅ is formed byconducting a heat oxidation process after polishing (buffing) of theupper chrome layer S₂. Due to the presence of the oxide film S₅ as theoutermost layer of the chrome plated part, corrosion resistance of theupper chrome layer S₂ itself can be improved, to thereby preventformation of white rust which is caused by corrosion of the chromelayer.

In the present invention, the oxide film may be formed solely fromCr₂O₃. Needless to say, when the oxide film contains not only Cr₂O₃, butalso a component other than Cr₂O₃ in a small amount, the oxide film isstill satisfactory in terms of strength.

In order to conduct polishing and heat oxidation, an apparatus such asshown in FIG. 7 can be employed. This apparatus comprises a primary lineL₁ of production; a centerless polishing disk apparatus 20 provided inthe primary line L₁; a secondary line L₂ of production provided inparallel to the primary line L₁; a pusher 21, a high-frequency coil 22and a cooling coil 23 provided in the secondary line L₂; and an inclinedstand-by member 24 connected to the primary line L₁ and the secondaryline L₂. The centerless polishing disk apparatus 20 comprises a buffwheel 20a and a regulating wheel 20b. After completion of the chromeplating process, the work W is polished between the buff wheel 20a andthe regulating wheel 20b of the centerless polishing disk apparatus 20and rolls on the inclined stand-by member 24 to the secondary line L₂,where the work W is continuously moved through the high-frequency coil22 and the cooling coil 23 by extension of a rod 21a of the pusher 21.Thus, polishing and heat oxidation can be efficiently conducted.

EXAMPLE 1

Using rods (diameter: 12.5 mm; length: 200 mm) made of steel (JIS S25C)as test pieces, and a chrome plating bath comprising 250 g/L of chromicacid, 2.5 g/L of sulfuric acid, 8 g/L of organic sulfonic acid and 10g/L of boric acid, pulse plating was conducted under the followingconditions: bath temperature=60° C.; maximum current density I_(U)=120A/dm²; minimum current density I_(L)=0 A/dm² (the same as in the case ofFIG. 2); pulse time (on-time) T₁ at maximum current density I_(U)=100 to800 μs; pulse time (off-time) T₂ at minimum current density I_(L)=100 to500 μs; and frequency=0.8 to 5.0 kHz. As a result, a crack-free lowerchrome layer S₁ (FIG. 1) having a thickness of about 3 μm was formed ona surface of each test piece. Subsequently, in the same chrome platingbath, general-purpose plating was conducted at a bath temperature of 60°C. and a current density of 60 A/dm². As a result, an upper chrome layerS₂ (FIG. 1) having a thickness of about 10 μm was formed on the lowerchrome layer S₁ on each test piece, to thereby obtain samples 2 to 18(as shown in Table 2). Further, for reference, using the same test pieceand chrome plating bath as mentioned above, general-purpose hard chromeplating was conducted at a bath temperature of 60° C. and a currentdensity of 60 A/dm². As a result, a single chrome layer having athickness of about 20 μm was formed on a surface of the test piece, tothereby obtain a sample 1.

With respect to the samples 2 to 18, a surface hardness (HV) wasmeasured and visual observation was made by using a microscope toevaluate formation of cracks in each of the lower and upper chromelayers S₁ and S₂ after deposition. Further, with respect to the lowerchrome layer S₁, residual stress and crystal grain size were measured asmentioned below. Further, the samples 2 to 18 were subjected to asalt-spray test in accordance with JIS Z2371, and visually observed toevaluate occurrence of rusting. With respect to the samples in which norusting was observed, they were subjected to heat treatment at 200° C.for 2 hours. The resultant samples were visually observed to evaluateformation of cracks on each of the lower and upper chrome layers S₁ andS₂ in the above-mentioned manner, and were subjected to the salt-spraytest in accordance with JIS Z2371 again to evaluate occurrence ofrusting. The color of a surface to each of the samples 2 to 18 wasobserved at the time of completion of formation of the lower chromelayer S₁. The above-mentioned measurements and observations were alsoconducted with respect to the single chrome layer of the sample 1.

Measurement of residual stress in the chrome layer was conducted by amethod called “X-Sen Ouryoku Sokuteihou (X-ray stress measurementmethod)” disclosed in “Hihakai Kensa (non-destructive inspection)”, vol.37, item 8, pages 636 to 642, edited by The Japanese Society forNon-destructive Inspection. Measurement of a crystal grain size of thechrome layer was conducted using an X-ray diffractometer, by using acharacteristic X-ray Cu-Kα (wavelength: 1.5405620 Å) with respect to theCr (222) diffraction plane. In this measurement, the crystal grain sizewas determined by assigning the result of measurement of the width(integral width) of a diffraction profile to the following Scherrer'sequation. As the integral width, a value corrected by a Cauchy functionwas used.D_(hkl)=K·λ/β₁cos θwherein

-   -   D_(hkl): crystal grain size (Å) [measured in a direction        perpendicular to (hkl)]    -   λ: wavelength of an X-ray for measurement (Å)    -   β₁: width (integral width) of a diffraction beam dependent on        the crystal grain size (rad)    -   θ: Bragg angle of the diffraction beam    -   K: constant (1.05)

Results of the above-mentioned measurements and observations are shownin Table 2.

TABLE 2 (1) Pulse time Crystal Cracking of Residual Hard- Appear-Rusting (μs) grain size S₁ after stress ness ance of Before heat Afterheat Eval- Sample No. T₁ T₂ of S₁ (nm) deposition (MPa) (HV) of S₁treatment treatment uation 1 (Comparative) 6.1 Observed +230 1,090Glossy Observed (2 h) NG 2 (Comparative) 100 100 7.8 Observed +276 1,034Glossy Observed (24 h) NG 3 (Comparative) 200 100 8.0 Observed +1601,017 Glossy Observed (24 h) NG 4 (Comparative) 150 150 8.2 Observed +10940 Glossy Observed (96 h) NG 5 (Comparative) 200 200 8.7 Not observed−65 920 Glossy Not observed (300 h) Observed (24 h) NG 6 (Presentinvention) 150 200 9.6 Not observed −150 870 Glossy Not observed (300 h)Not observed (300 h) OK 7 (Present invention) 100 200 9.8 Not observed−203 835 Glossy Not observed (300 h) Not observed (300 h) OK 8 (Presentinvention) 110 220 10.1 Not observed −220 840 Glossy Not observed (300h) Not observed (300 h) OK 9 (Present invention) 800 300 10.5 Notobserved −205 818 Glossy Not observed (300 h) Not observed (300 h) OK 10(Present invention) 400 300 10.6 Not observed −305 782 Glossy Notobserved (300 h) Not observed (300 h) OK 11 (Present invention) 200 30011.1 Not observed −339 742 Glossy Not observed (300 h) Not observed (300h) OK 12 (Present invention) 300 300 11.7 Not observed −313 710 GlossyNot observed (300 h) Not observed (300 h) OK 13 (Present invention) 600400 12.3 Not observed −323 681 Glossy Not observed (300 h) Not observed(300 h) OK 14 (Present invention) 500 400 13.5 Not observed −334 630Glossy Not observed (300 h) Not observed (300 h) OK 15 (Presentinvention) 400 400 15.4 Not observed −272 602 Glossy Not observed (300h) Not observed (300 h) OK 16 (Comparative) 300 400 16.0 Observed +30546 Milky Observed (96 h) NG 17 (Comparative) 600 500 16.7 Observed +53498 Milky Observed (96 h) NG 18 (Comparative) 700 500 18.1 Observed +18450 Milky Observed (96 h) NG

As shown in Table 2, with respect to the sample 1 (comparative) obtainedby general-purpose hard chrome plating, the chrome layer contained manycracks and rusting was observed over an entire surface of the chromelayer at an extremely early time (2 hours) in the salt-spray test.

The samples 2 to 18 were obtained by the two-step plating process. Ofthese, with respect to the samples 2 to 4 and 16 to 18 (comparative), atthe time of completion of the plating process, the upper chrome layer S₂contained many cracks and the lower chrome layer S₁ was also cracked.When the samples 2 to 4 and 16 to 18 were subjected to the salt-spraytest after the plating process, rusting was observed at a relativelyearly time (24 to 96 hours) in the salt-spray test. Thus, with respectto the samples 2 to 4 and 16 to 18, rusting occurred in the salt-spraytest before heat treatment. Therefore, no heat treatment was conductedwith respect to these samples.

On the other hand, with respect to the samples 5 to 15 also obtained bythe two-step plating process, at the time of completion of the platingprocess, the upper chrome layer S₂ contained many cracks, but nocracking was observed in the lower chrome layer S₁. Further, withrespect to the samples 5 to 15, no rusting was observed until 300 hoursafter the start of the salt-spray test.

With respect to the samples 5 to 15 in which no rusting was observedbefore heat treatment, they were subjected to heat treatment at 200° C.for 2 hours and visually observed to evaluate formation of cracks andoccurrence of rusting. With respect to the sample 5 (comparative),cracking was observed in the lower chrome layer S₁ and rusting occurredat a relatively early time (24 hours) in the salt-spray test. On theother hand, with respect to the samples 6 to 15 (present invention), nocracking was observed in the lower chrome layer S₁ even after heattreatment and no rusting was observed until 300 hours after the start ofthe salt-spray test.

Comparison was made between the samples 1 to 18 with respect to residualstress in the lower chrome layer S₁ (the single chrome layer in the caseof the sample 1). With respect to the samples 1 to 4 and 16 to 18(comparative), the residual stress was tensile residual stress. Withrespect to the samples 5 to 15, the residual stress was compressiveresidual stress. Especially, the samples 6 to 15 (present invention) hada large compressive residual stress of 150 MPa or more.

Further, comparison was made between the samples 1 to 18 with respect toa crystal grain size of the lower chrome layer S₁ (the single chromelayer in the case of the sample 1). With respect to the samples 1 to 5(comparative), the crystal grain size was less than 9 nm. With respectto the samples 6 to 18, the crystal grain size was 9 nm or more. In eachof the samples 16 to 18, the chrome layer had an especially largecrystal grain size of 16 nm or more.

With respect to the surface hardness (HV), the surface hardness of thesample 1 (obtained by general-purpose hard plating) was the highest.With respect to the remaining samples, the larger the crystal grainsize, the lower the surface hardness.

Further, comparison was made between the samples 1 to 18 with respect tothe color of a surface of the lower chrome layer S₁ (with single chromelayer in the case of the sample 1). With respect to the samples 1 to 15,the chrome layer had a glossy surface characteristic of chrome plating.With respect to the samples 16 to 18, the chrome layer had a milkysurface.

From the above, it is apparent that formation of cracks in the chromelayer is dependent on the residual stress and the crystal grain size ofthe chrome layer. In order to ensure a desired corrosion resistance ofthe chrome plated part by suppressing cracking of the chrome layer evenwhen it is subject to thermal hysteresis, it is necessary to conduct thechrome plating process so that the lower chrome layer S₁ having acompressive residual stress of 150 MPa or more, and preferably having acrystal grain size of 9 nm or more can be obtained. The compressiveresidual stress which can be obtained solely by adjusting the waveformof a pulse current is limited. Therefore, an appropriate waveform of apulse current must be selected, depending on the intended applicationsof the chrome plated part. With respect to the crystal grain size, thelower chrome layer of each of the samples 16 to 18, which had a crystalgrain size of 16 nm or more, had tensile residual stress. Therefore, itis preferred that the crystal grain size be less than 16 nm.

EXAMPLE 2

Using the same test piece and chrome plating bath as used in Example 1,pulse plating was conducted under the following conditions: bathtemperature=60° C.; maximum current density I_(U)=120 A/dm²; minimumcurrent density I_(L)=0 A/dm²; pulse time (on-time) T₁ at maximumcurrent density I_(U)=1,400 μs; pulse time (off-time) T₂ at minimumcurrent density I_(L)=600 μs; and frequency=500 Hz. As a result, a lowerchrome layer S₁ (FIG. 4) having a thickness of about 2 μm was formed ona surface of the test piece. Subsequently, in the same chrome platingbath, pulse plating was conducted under the following conditions: bathtemperature=60° C.; maximum current density I_(U)=120 A/dm²; minimumcurrent density I_(L)=0 A/dm²; on-time T₁=1,400 μs; off-time T₂=400 μs;and frequency=625 Hz. As a result, an intermediate chrome layer S₃ (FIG.4) having a thickness of about 2 μm was formed on a surface of the lowerchrome layer S₁. Subsequently, in the same chrome plating bath, pulseplating was conducted under the following conditions: bathtemperature=60° C.; maximum current density I_(U)=120 A/dm²; minimumcurrent density I_(L)=0 A/dm²; on-time T₁=200 μs; off-time T₂=100 μs;and frequency=3,333 Hz. As a result, an intermediate chrome layer S₄(FIG. 4) having a thickness of about 2 μm was formed on a surface of theintermediate chrome layer S₃. Subsequently, in the same chrome platingbath, general-purpose plating was conducted at a bath temperature of 60°C. and a current density of 60 A/dm². As a result, an upper chrome layerS₂ (FIG. 4) having a thickness of about 5 μm was formed on theintermediate chrome layer S₄, to thereby obtain a sample.

With respect to the obtained sample, each of the lower chrome layer S₁,the intermediate chrome layers S₃ and S₄ and the upper chrome layer S₂was visually observed by using a microscope to evaluate formation ofcracks. Further, by the same methods as mentioned above in Example 1,residual stress and crystal grain size were measured with respect toeach of the chrome layers S₁ to S₄. The sample was subjected to thesalt-spray test in accordance with JIS Z2371, and visually observed toevaluate occurrence of rusting. After the salt-spray test, the samplewas subjected to heat treatment at 200° C. for 2 hours, and subjected tothe salt-spray test in accordance with JIS Z2371 again. The resultantsample was visually observed to evaluate occurrence of rusting. Resultsof the above-mentioned measurements and observations are shown in Table3.

TABLE 3 Crystal Rusting Residual grain Cracking Before After Chromestress size after heat heat layer (MPa) (nm) deposition treatmenttreatment S₁ −279 12.2 Not observed Not observed Not observed S₃ −16310.7 Not observed S₄ +226 8.0 Slightly observed S₂ +300 6.6 Observed

As shown in Table 3, no cracking was observed with respect to the lowerchrome layer S₁ and the intermediate chrome layer S₃. The intermediatechrome layer S₄ on a side of the upper chrome layer S₂ was slightlycracked and the upper chrome layer S₂ contained many cracks. Withrespect to the residual stress, each of the lower chrome layer S₁ andthe intermediate chrome layer S₃ had compressive residual stress aslarge as more than 150 MPa. Each of the intermediate chrome layer S₄ andthe upper chrome layer S₂ had tensile residual stress. With respect tothe crystal grain size, the crystal grain size of each of the lowerchrome layer S₁ and the intermediate chrome layer S₃ was as large asmore than 9 nm. The crystal grain size of each of the intermediatechrome layer S₄ and the upper chrome layer S₂ was much smaller than 9nm.

No rusting was observed in the salt-spray test before and after heattreatment. Therefore, it was understood that the sample had sufficientcorrosion resistance.

EXAMPLE 3

Using the same test pieces and chrome plating bath as used in Example 1,pulse plating was conducted under the following conditions: bathtemperature=60° C.; maximum current density I_(U)=120 A/dm²; minimumcurrent density I_(L)=0 A/dm²; pulse time (on-time) T₁ at maximumcurrent density I_(U)=300 μs; pulse time (off-time) T₂ at minimumcurrent density I_(L)=300 μs; and frequency: 1.7 kHz. As a result, acrack-free lower chrome layer S₁ (FIG. 1) having a thickness of about 3μm was formed on a surface of each test piece. Subsequently, in the samechrome plating bath, general-purpose plating was conducted at a bathtemperature of 60° C. and a current density of 60 A/dm². As a result, acracked upper chrome layer S₂ (FIG. 1) having a thickness of about 10 μmwas formed on the lower chrome layer S₁ on each test piece. The upperchrome layer S₂ was finished by buffing so as to have a surfaceroughness Ra of 0.08 μm. As a result, samples 31 and 32 were obtained.The sample 31 was subjected to a general-purpose baking process at 210°C. for 4 hours, to thereby form an oxide film (containing Cr₂O₃ as amain component) on the upper chrome layer S₂. The sample 32 wassubjected to high-frequency heating at a maximum heating temperature of400° C. for a short period of time (about 10 seconds), to thereby forman oxide film (containing Cr₂O₃ as a main component) on the upper chromelayer S₂.

For comparison, using the same test piece and chrome plating bath asused in Example 1, pulse plating was conducted under the followingconditions: bath temperature=60° C.; maximum current density I_(U)=120A/dm²; minimum current density I_(L=)0 A/dm²; on-time T₁=200 μs;off-time T₂=200 μs; and frequency=2.5 kHz. As a result, a crack-freelower chrome layer S₁ having a thickness of about 3 μm was formed on asurface of the test piece. Subsequently, in the same chrome platingbath, general-purpose plating was conducted at a bath temperature of 60°C. and a current density of 60 A/dm². As a result, a cracked upperchrome layer S₂ having a thickness of about 10 μm was formed on asurface of the lower chrome layer S₁, to thereby obtain a sample 33. Thesample 33 was subjected to the above-mentioned buffing andhigh-frequency heating. Further, for comparison, substantially the sameprocedure for obtaining the sample 31 was repeated, except that thebaking process was conducted before buffing, to thereby obtain a sample34.

With respect to each of the samples 31 to 34, residual stress andcrystal grain size of the lower chrome layer S₁ were measured by thesame methods as mentioned above in Example 1. The samples 31 to 34 weresubjected to the salt-spray test in accordance with JIS Z2371, andvisually observed to evaluate formation of red rust and white rust.Results of the above-mentioned measurements and observations are shownin Table 4.

TABLE 4 Crystal grain Residual Sample size of stress of Method of heatRusting No. Process S₁ (nm) S₁ (MPa) oxidation White rust Red rust 31Plating- 11.7 −313 Baking Not Not Polishing- observed observed Oxidation32 Plating- 11.7 −313 High- Not Not Polishing- frequency observedobserved Oxidation heating 33 Plating- 8.7 −65 High- Not ObservedPolishing- frequency observed Oxidation heating 34 Plating- 8.7 −65Baking Observed Not Oxidation- observed Polishing

As shown in Table 4, in each of the samples 31 and 32, the lower chromelayer S₁ had sufficiently large compressive residual stress and asufficiently large crystal grain size. On the other hand, in each of thesamples 33 and 34, the lower chrome layer S₁ had undesirably lowcompressive residual stress and an undesirably small crystal grain size.

After the salt-spray test, with respect to each of the samples 31 and 32(present invention), red rust which forms due to corrosion of a metallicsubstrate and white rust which forms due to corrosion of the chromelayer were not observed. On the other hand, red rust was observed in thesample 33 (comparative) and white rust was observed in the sample 34(comparative). Red rust was observed in the sample 33 because both ofthe lower chrome layer S₁ and the upper chrome layer S₂ containedcracks. White rust was observed in the sample 34 because the oxide filmformed by the baking process was removed by buffing. FIG. 8 is amicrophotograph showing white rust formed in the sample 34. No red rustwas observed in the sample 34 because, during buffing, the cracks wereclogged due to the occurrence of plastic flow in the chrome layer.

FIG. 9 is a graph showing a relationship between the thickness ofplating and residual stress in the chrome plated part of the presentinvention when pulse plating is conducted by application of the samepulse current as used for obtaining the sample 12. In the graph, thereis substantially no stress gradient such as that shown in theabove-mentioned Examined Japanese Patent Application Publication No.43-20082. Average compressive residual stress is stably maintained at alevel of 100 MPa or more.

As has been described above, the chrome plated part of the presentinvention maintains excellent corrosion resistance even when it issubject to thermal hysteresis. Therefore, the present invention isadvantageous when applied to products used in corrosive environments andunder high temperature conditions. The chrome plated part of the presentinvention is especially advantageous when it comprises a crack-freechrome layer provided as the lowermost chrome layer and a cracked chromelayer provided as the uppermost chrome layer, because such a chromeplated part has excellent wear resistance and excellent slidingproperties.

In the chrome plating method of the present invention, compressiveresidual stress and crystal grain size of the chrome layer can be easilycontrolled by adjusting the waveform of a pulse current. Therefore, achrome plated part having desired properties can be efficientlyobtained.

Further, in the method of the present invention for producing a chromeplated part, an oxide film containing Cr₂O₃ may be formed as anoutermost layer of the chrome plated part. Therefore, formation of redrust due to corrosion of a metallic substrate and formation of whiterust due to corrosion of the chrome layer can be surely prevented.

The present invention can be applied to a surface of a piston rod for ashock absorber or a surface of a piton ring for an engine.

The entire disclosures of Japanese Patent Application Nos. 10-332047 and11-285503 filed on Nov. 6, 1998 and Oct. 6, 1999, respectively, eachincluding a specification, claims, drawings and summary are incorporatedherein by reference in their entirety.

1. A chrome plated part comprising a substrate having a crack-freechrome layer on a surface thereof, the crack-free chrome layer havingcompressive residual stress of 100 MPa or more and being formed byelectroplating.
 2. A chrome plated part according to claim 1 comprisinga substrate having a crack-free chrome layer on a surface thereof, thecrack-free chrome layer having compressive residual stress of 100 MPa ormore and being formed by electroplating with a pulse current, whereinthe chrome layer has a crystal grain size of 9 nm or more.
 3. A chromeplated part according to claim 2, wherein the crystal grain size of thechrome layer is from 9 nm to less than 16 nm.
 4. A chrome plated partaccording to claim 1 claim 2, wherein the crack-free chrome layer is alower chrome layer and the chrome plated part further comprises acracked upper chrome layer which is formed on the lower chrome layer byelectroplating.
 5. A chrome plated part according to claim 4, whereinthe upper chrome layer has tensile residual stress.
 6. A chrome platedpart according to claim 5 comprising a substrate having a crack-freechrome layer on a surface thereof, the crack-free chrome layer havingcompressive residual stress of 100 MPa or more and being formed byelectroplating with a pulse current, wherein: the crack-free chromelayer is a lower chrome layer and the chrome plated part furthercomprises a cracked upper chrome layer which is formed on the lowerchrome layer by electroplating with a pulse current; the upper chromelayer has tensile residual stress; and the upper chrome layer has acrystal grain and the crystal grain has a size less than 9 nm.
 7. Achrome plated part according to claim 4, comprising a substrate having acrack-free chrome layer on a surface thereof, the crack-free chromelayer having compressive residual stress of 100 MPa or more and beingformed by electroplating with a pulse current, wherein the crack-freechrome layer is a lower chrome layer and the chrome plated part furthercomprises a cracked upper chrome layer which is formed on the lowerchrome layer by electroplating with a pulse current, the chrome platedpart further comprising at least one intermediate chrome layer which isformed between the lower chrome layer and the upper chrome layer byelectroplating.
 8. A chrome plated part according to any one of claims1, 4 and 7, further comprising comprising a substrate having acrack-free chrome layer on a surface thereof, the crack-free chromelayer having compressive residual stress of 100 MPa or more and beingformed by electroplating with a pulse current and an oxide filmcontaining Cr₂O₃ as an outermost layer thereof.
 9. A chrome plated partcomprising a substrate having a crack-free chrome layer on a surfacethereof, the crack-free chrome layer having compressive residual stressof 150 MPa or more and being formed by electroplating.
 10. A chromeplated part according to claim 9 comprising a substrate having acrack-free chrome layer on a surface thereof, the crack-free chromelayer having compressive residual stress of 150 MPa or more and beingformed by electroplating with a pulse current, wherein the crack-freechrome layer has a crystal grain size of 9 nm or more.
 11. A chromeplated part according to claim 10, wherein the crystal grain size of thecrack-free chrome layer is from 9 nm to less than 16 nm.
 12. A chromeplated part comprising: a substrate having a surface; and a chrome layerdeposited on the surface of the substrate by electroplating, the chromelayer having compressive residual stress of 100 MPa or more.
 13. Achrome plating method comprising the step of conducting electroplatingof a work in a chrome plating bath by application of a pulse current,the chrome plating bath containing organic sulfonic acid, to therebydeposit a crack-free chrome layer on a surface of the work, thecrack-free chrome layer having compressive residual stress of 150 MPa ormore.
 14. A chrome plating method comprising the step of conductingelectroplating of a work in a chrome plating bath by application of apulse current, the chrome plating bath containing organic sulfonic acid,to thereby deposit a crack-free chrome layer on a surface of the work,the crack-free chrome layer having compressive residual stress of 100MPa or more.
 15. A chrome plating method according to claim 14 or 13,wherein the crack-free chrome layer is formed to have a crystal grainsize of from 9 nm to less than 16 nm by adjusting a waveform of thepulse current.
 16. A method for producing a chrome plated part,comprising the steps of: conducting the chrome plating method of claim14; polishing the crack-free chrome layer on the surface of the work;and conducting heat oxidation, to thereby form an oxide film containingCr₂O₃ on a surface of the crack-free chrome layer.
 17. A methodaccording to claim 16, wherein the heat oxidation is conducted under thesame conditions as conditions of a baking process.
 18. A methodaccording to claim 16, wherein the heat oxidation is conducted byhigh-frequency heating.
 19. A chrome plating method according to claim14, further comprising the step of conducting, after the pulse plating,electroplating of the work in the same chrome plating bath as the chromeplating bath for the pulse plating, by one of adjustment of a waveformof the pulse current and application of a direct current, to therebydeposit a cracked upper chrome layer on the crack-free chrome layer. 20.A chrome plating method according to claim 14, further comprising thesteps of: conducting, after the pulse plating, electroplating of thework in the same chrome plating bath as the chrome plating bath for thepulse plating, by one of adjustment of a waveform of the pulse currentand application of a direct current, to thereby deposit an intermediatechrome layer on the crack-free chrome layer; and conductingelectroplating of the work in the same chrome plating bath as the chromeplating bath for the pulse plating, by one of adjustment of the waveformof the pulse current and application of the direct current, to therebydeposit a cracked upper chrome layer on the intermediate chrome layer.21. A chrome plating method according to claim 19 or 20, wherein thechrome layers are deposited by continuous operation by continuouslymoving the work in the chrome plating bath.
 22. A chrome plating methodaccording to claim 19 or 20, wherein the chrome layers are deposited bybatchwise operation by immersing the work in the chrome plating bath.23. A method for producing a chrome plated part, comprising the stepsof: conducting the chrome plating method of claim 19 or 20; polishingthe upper chrome layer formed on the crack-free chrome layer on thesurface of the work; and conducting heat oxidation, to thereby form anoxide film containing Cr₂O₃ on a surface of the upper chrome layer. 24.A method according to claim 23, wherein the heat oxidation is conductedunder the same conditions as conditions of a baking process.
 25. Amethod according to claim 23, wherein the heat oxidation is conducted byhigh-frequency heating.
 26. A chrome plating method comprising the stepsof: providing a substrate having a surface; and depositing a chromelayer on the surface of the substrate by electroplating so that thechrome layer has compressive residual stress of 100 MPa or more.
 27. Themethod according to claim 17, wherein said baking process is at 191±14°for 3 hours or more.
 28. A method of chrome plating comprising the stepsof: providing a substrate having a surface; depositing a crack-freechrome layer on the surface of the substrate by electroplating byapplication of a pulse current so that the crack-free chrome layer hascompressive residual stress of 100 MPa or more, wherein saidelectroplating is in the presence of an organic sulfonic acid, andforming an oxide film containing Cr ₂ O ₃ on the surface of thecrack-free chrome layer as an outermost layer.
 29. A chrome plated partaccording to claim 7, further comprising an oxide film containing Cr₂ O₃ as an outermost layer thereof.
 30. A chrome plated part comprising asubstrate having a crack-free chrome layer on a surface thereof, thecrack-free chrome layer having a compressive residual stress of 100 MPaor more and being formed by electroplating with a pulse current,wherein: the crack-free chrome layer is a lower chrome layer and thechrome plated part further comprises a cracked upper chrome layer whichis formed on the lower chrome layer by electroplating with a pulsecurrent; and the chrome plated part further comprises an oxide filmcontaining Cr ₂ O ₃ as an outermost layer thereof.